Flash memory

ABSTRACT

A MONOS Charge-Trapping flash (CTF), with record thinnest 3.6 nm ENT trapping layer, has a large 3.1 V 10-year extrapolated retention window at 125° C. and excellent 10 6  endurance at a fast 100 μs and ±16 V program/erase. This is achieved using As + -implanted higher κ trapping layer with deep 5.1 eV work-function of As. In contrast, the un-implanted device only has a small 10-year retention window of 1.9 V at 125° C. A MoN—[SiO 2 —LaAlO 3 ]—[Ge—HfON]—[LaAlO 3 —SiO 2 ]—Si CTF device is also provided with record-thinnest 2.5-nm Equivalent-Si 3 N 4 -Thickness (ENT) trapping layer, large 4.4 V initial memory window, 3.2 V 10-year extrapolated retention window at 125° C., and 3.6 V endurance window at 10 6  cycles, under very fast 100 μs and low ±16 V program/erase. These were achieved using Ge reaction with HfON trapping layer for better charge-trapping and retention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to flash memories, and, more particularly, to a flash memory having a charge-trapping layer formed by implanting arsenic into ZrON.

2. Description of Related Art

According to International Technology Roadmap for Semiconductors (ITRS), the degraded endurance and retention are the toughest challenges to further down-scaling the Charge-Trapping Flash (CTF), due to the fewer electrons stored in highly scaled device. On the other hand, scaling down the Si₃N₄ charge-trapping layer to 3-4 nm is needed in ITRS for continuous device scaling, but no proposed solution up to now. However, this worsens the retention and endurance due to the poorer trapping capability at thinner Si₃N₄, where nearly no charge trapping was found in 2 nm Si₃N₄. Although the retention is improved by using a thicker tunnel oxide, this yields reduced erase speed. Such retention and erase-speed trade-off is a basic limitation of CTF.

Previously we addressed this limitation with a deep trapping energy E_(vac)-E_(C) Al(Ga)N or HfON in a metal-oxide-nitride-oxide-Si (MONOS) device. The better retention of high-κ Al(Ga)N MONOS CTF was also listed in ITRS. One drawback of desired higher κ HfON is the lower trapping efficiency; thus, the double trapping HfON—Si₃N₄ CTF was used. Yet the scaling equivalent-Si₃N₄-thickness (ENT) is still limited to 7 nm.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems of the prior art, it is a primary objective of the present invention to provide flash memory that has a charge-trapping layer formed by implanting arsenic into ZrON.

In an embodiment of the present invention, the flash memory includes: a substrate; a first SiO₂ layer formed on the substrate; a first high-κ layer formed on the first SiO₂ layer; a metal-implanted oxynitride layer formed on the first high-κ layer; a second high-κ layer formed on the metal-implanted oxynitride layer; a second SiO₂ layer formed on the second high-κ layer; and a gate layer formed on the second SiO₂ layer.

In another embodiment of the present invention, the flash memory includes: a Si substrate; a first SiO₂ layer formed on the Si substrate; a first high-κ layer formed on the first SiO₂ layer; a first HfON layer formed on the first high-κ layer; a first Ge/oxynitride layer formed on the oxynitride layer; a second high-κ layer formed on the Ge/oxynitride layer; a second SiO₂ layer formed on the second high-κ layer; and a gate layer formed on the second SiO₂ layer.

The present invention further provides a method of fabricating a flash memory, including: providing a substrate; forming a first SiO₂ layer on the substrate; forming a first high-κ layer on the first SiO₂ layer; forming a metal-implanted oxynitride layer on the first high-κ layer; forming a second high-κ layer on the metal-implanted oxynitride layer; forming a second SiO₂ layer on the second high-κ layer; and forming a TaN layer on the second SiO₂ layer.

In an embodiment, the As-implanted ZrON is formed by implanting ZrON with As at 60°-tilted angle, 3 KeV and 5×10¹⁵ cm⁻² dose, and followed by 950° C. RTA.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein:

FIGS. 1( a) to (c) are schematic band diagrams of a traditional MONOS CTF, a double Si₃N₄-HfON trapping CTF, and a As⁺-implanted ZrON CTF, respectively;

FIG. 2 shows J_(g)-V_(g) curves of ZrON MONOS nonvolatile memory (NVM) devices with and without As⁺-implant;

FIG. 3 shows C-V hysteresis of ZrON MONOS NVM devices with and without As⁺-implant;

FIG. 4 shows program characteristics of control ZrON MONOS NVM devices for different voltages and times;

FIG. 5 shows erase characteristics of control ZrON MONOS NVM devices for different voltages and times;

FIG. 6 shows program characteristics of As⁺-implanted ZrON MONOS NVM devices for different voltages and times;

FIG. 7 shows erase characteristics of As⁺-implanted ZrON MONOS NVM devices for different voltages and times;

FIG. 8 is a schematic structure of an As⁺-implanted ZrON/LaAlO₃/SiO₂/Si of an embodiment according to the present invention;

FIG. 9 shows XRD of control and As⁺-implanted ZrON/LaAlO₃/SiO₂/Si structure after 950° C. RTA;

FIG. 10 shows SIMS of the As⁺-implanted ZrON/LaAlO₃/SiO₂/Si structure after 950° C. RTA;

FIG. 11 shows retention characteristics of control un-implanted ZrON MONOS NVM devices at 25-125° C.;

FIG. 12 shows retention characteristics of As⁺-implanted ZrON MONOS NVM devices at 25-125 ° C.;

FIG. 13 shows endurance characteristics of As⁺-implanted ZrON MONOS NVM devices;

FIGS. 14( a) and (b) are a schematic structure and s schematic energy band diagram of a Ge/HfON CTF memory of an embodiment according to the present invention, respectively;

FIGS. 15( a) and (b) show program and erase characteristics of HfON CTF memory with and without Ge for different voltages and times; and

FIGS. 16( a) and (b) show retention characteristics of a flash memory and cycling characteristics of HfON CTF memory with and without Ge.

DETAILED DESCRIPTION OF THE INVENTION

The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects can be apparently understood by those in the art after reading the disclosure of this specification. The present invention can also be performed or applied by other different embodiments. The details of the specification may be on the basis of different points and applications, and numerous modifications and variations can be devised without departing from the spirit of the present invention.

High performance MONOS CTF with highly scaled 3.6 nm ENT is reached and meets ITRS scaling target for the first time. At 125° C. and ±16 V program/erase (P/E), the device has fast 100 μs speed and large extrapolated 10-year retention of 3.1 V. The excellent results were reached using metallic Arsenic (As) implant into higher κ ZrON (κ=35) as trapping layer. In contrast, small 10-year retention window of 1.9 V is found in control ZrON CTF. The improved memory window is due to the better electron trapping capability of As⁺-implanted ZrON. The excellent 10⁶ cycles and good 125° C. retention may be ascribed to the deep E_(vac)-E_(C) ZrON and 5.1 eV work-function (Φ_(m)) of metallic As, to minimize the Schottky emission and tunnel leakage. The excellent 10⁶ cycles are vitally important to allow further endurance improvement in highly scaled CTF device with fewer electrons. These results compare well with other reported data listed in Table 1, with the smallest 3.6 nm ENT, fast 100 μs speed, large memory window, good retention at 125° C. and the best 10⁶ endurance.

TABLE 1 ΔV_(th) (V) for ΔV_(th) (V) for P/E conditions 10-year 10-year for retention & Initial retention retention ΔV_(th) (V) cycling ΔV^(th) (V) @ 85° C. @ 125° C. @ Cycles This work 16 V 100 μs/-16 V 4.9 3.4 3.1 4.3 @ 10⁶ (As⁺-implanted) 100 μs This work (control 16 V 100 μs/-16 V 2.9 2.1 1.9 — un-implanted) 100 μs TANOS 13.5 V 100 μs/-13 V 4.4 2.07 —   4 @ 10⁵ SiO₂/Si₃N₄/Al₂O₃/TaN 10 ms Tri-gate 11.5 V 3 ms/-11.5 V 1.2 1.1 (@ 25° C.) — 1.5 @ 10⁴ SiO₂/Si₃N₄/SiO₂ 100 ms FinFET 13 V 10 μs/-12 V 4.5 2.4 — 3.5 @ 10⁴ SiO₂/Si₃N₄/SiO₂ 1 ms SiO₂/AlGaN/AlLaO₃ 11 V 100 μs/-11 V 3.0 1.6 — 2.3 @ 10⁵ 100 μs

The TaN-[SiO₂—LaAlO₃]—ZrON—[LaAlO₃—SiO₂]-Si CTF device has 2.5 nm thermal SiO₂, 2.5 nm LaAlO₃, 18 nm ZrON_(0.2), 8 nm LaAlO₃, 6 nm LPCVD SiO₂, and 200 nm TaN. The LaAlO₃, ZrON_(0.2), and TaN were deposited by physical vapor deposition (PVD). To improve the trapping capability, the ZrON was implanted by As at 60°-tilted angle, 3 KeV and 5×10¹⁵ cm⁻² dose, followed by 950° C. RTA to reduce the ion-implanted damage. After gate definition, self-aligned As implant is applied and RTA is used to activate the dopants. The LaAlO₃ was from binary Al₂O₃ and La₂O₃, used for V_(t) tuning in 32 nm gate-first high-κ p- and n-MOSFETs, respectively. For comparison, control CTF device was also fabricated without the As⁺-implant into ZrON.

A. P/E Characteristics:

FIG. 1 shows the traditional, double trapping Si₃N₄-HfON, and As⁺-implanted ZrON MONOS CTF devices. The As⁺-implanted ZrON has higher κ than HfON and deep Φ_(m) of As for trapping. FIG. 2 shows the gate current (J_(g)) of CTF. Close J_(g) of As⁺-implanted device with control is reached. Larger C-V hysteresis of 8.1 V was obtained in As⁺-implanted CTF than control device under ±16 V sweep (FIG. 3). FIGS. 4 and 5 show the program and erase data in control devices. A small ΔV_(th) memory window of 2.9 V was measured at 100 μs at ±16 V P/E that is typical for metal-oxide-nitride trapping MONOS with low trapping efficiency. The ΔV_(th) is significantly larger for As⁺-implanted CTF devices shown in FIGS. 6 and 7, with a large ΔV_(th) window of 4.9 V at ±16 V 100 μs is P/E. This indicates the better trapping efficiency in As⁺-implanted ZrON that may be due to metallic As atoms and implant-created defects. Since a 950° C. RTA is applied to lower the implanted defects, the As atoms may play a major role for better trapping. The fast 100 μs P/E speed is due to the existing ΔE_(C) and ΔE_(V) in LaAlO₃/ SiO₂ for easier tunneling, where the larger physical thickness improves the retention with only 3 nm EOT in tunnel oxide.

B. Characterization of As⁺-implanted ZrON:

The As⁺-implanted ZrON has a schematic structure shown in FIG. 8, which was analyzed by XRD and SIMS shown in FIGS. 9 and 10. As shown in FIG. 8, a flash memory 10 of an embodiment according to the present invention comprises a substrate 102, a SiO₂ layer 104 formed on the substrate 102, a LaAlO₃ layer 106 formed on the SiO₂ layer 104, an As-implanted ZrON layer 108 formed on the LaAlO₃ layer 106, another LaAlO₃ layer 110 formed on the As-implanted ZrON layer 108, another SiO₂ layer 112 formed on the another LaAlO₃ layer 110, and a TaN layer 114 formed on the another SiO₂ layer 112. In an embodiment, the SiO₂ layer 104 is 2.5 nm thick; the LaAlO₃ layer 106 is 2.5 nm thick; the As-implanted ZrON layer 108 is 18 nm thick; the another LaAlO₃ layer 110 is 8 nm thick; the another SiO₂ layer 112 is 6 nm thick; and the TaN layer 114 is 200 nm thick. The ZrON poly-grains are found in X-TEM that gives the higher κ value and smaller ENT. The XRD shows weak As peaks with the same angle of clustered As-dots in As-rich GaAs, which suggests the forming small As metal dots in ZrON although beyond our TEM resolution. Such metallic As with deep 5.1 eV Φ_(m) inside ZrON traps may also reduce the Schottky emission and tunnel leakage, in addition to the large E_(vac)-E_(C) of ZrON. From SIMS, the As concentration at 60° 3-keV implant reduces rapidly with thickness and mainly within ZrON that explains the close J_(g) with control device in FIG. 2.

C. Retention & Endurance:

FIGS. 11 and 12 show the retention data of As⁺-implanted and control ZrON CTF devices at 25, 85 and 125° C. A large 10-year extrapolated window of 3.1 V is measured at 125° C. in As⁺-implanted devices at 100 μs and ±16 V P/E. This is significantly better than the 1.9 V 10-year window in control devices. Such large 10-year retention data allows multi-level cells (MLC) storage even at 125° C. The good retention is due to the extra ΔE_(C) confinement energy in LaAlO₃/ZrON/LaAlO₃ and also deep Φ_(m) of metallic As shown in FIG. 1( c), while fast 100 μs erase is also reached from the lowered hole energy barrier ΔE_(V) in LaAlO₃/SiO₂ tunnel oxide. The fast P/E speed and lowered hole tunnel barrier ΔE_(V) lead to excellent endurance: as shown in FIG. 13, a still large 4.3 V window is obtained even at 10⁶ P/E cycles. Such excellent cycling data are vitally important and allow further improving the endurance in highly scaled CTF device with fewer stored electrons. Table 1 compares various MONOS CTF devices. The novel device compares well with other devices, with the record thinnest 3.6 nm ENT trapping layer, large memory window, good 125° C. retention, fast 100 μs P/E speed, and the highest 10⁶ endurance.

Using low energy As⁺-implant into higher κ ZrON trapping layer, this novel CTF device shows excellent device performance of highly scaled 3.6 nm ENT, large 10-year extrapolated retention window of 3.1 V at 125° C. and 1 million times endurance, at a fast 100 μs and low ±16V P/E.

Among various types of NVM, the flash memory has irreplaceable merits of the lowest switching energy and excellent device distribution that are vital for high-density sub-Tb memory arrays. To continue downscale into sub-20-nm, the MONOS CTF devices are proposed to replace the poly-Si floating-gate (FG) flash memory according to ITRS. This is due to the discrete charge-trapping property, simple planar structure, and small cell-cell disturbance that are needed for three-dimensional (3D) flash memory.

One challenge for CTF is the small confinement energy between Si₃N₄ trapping layer and SiO₂ barrier that degrades the high temperature retention. To address this issue, deep E_(C) high-κ AlGaN and HfON were used to replace the Si₃N₄, which was listed in ITRS for continuous downscaling. In addition, we improved the charge-trapping efficiency at an ENT of 3.6-nm, by using As⁺-implanted high-κ trapping layer to reach a large 10-year retention window of 3.1 V at 125° C. However, further downscaling the ENT is limited by the ion-implanted damage to tunnel oxide.

In this context, a high performance CTF memory at a record thinnest 2.5-nm ENT trapping layer is disclosed for the first time. This device has a large extrapolated 10-year retention memory window of 3.2 V at 125° C. and excellent endurance of 10⁶ cycles, under fast 100 μs and low ±16 V P/E pulses. These were achieved using Ge reaction with HfON trapping layer to form the HfGeON, even at ultra-thin 2.5-nm ENT. Such excellent device integrity is unreachable for conventional Si₃N₄ CTF device due to nearly no trapping at 2-nm Si₃N₄.

EXPERIMENTS

The MoN—[SiO₂—LaAlO₃]—[Ge—HfON]—[LaAlO₃—SiO₂]—Si MONOS CTF devices were made on standard 6-in p-type Si wafers. The double tunnel oxide layers of 2.7-nm thick thermal SiO₂ was first grown on Si substrates and followed by depositing 2.5-nm thick LaAlO₃ by physical vapor deposition (PVD). Then the charge trapping layers of 8-nm thick HfON and 1.5-nm thick Ge were deposited by PVD. Next, a 6.5-nm thick LaAlO₃ was deposited by PVD and 6.5-nm thick SiO₂ was deposited by chemical vapor deposition (CVD) using Tetraethyl orthosilicate (Si(C₂H₅O)₄) to form the double blocking layers. Finally, a 200-nm thick MoN was deposited by PVD, followed by gate definition, reactive ion-etching (RIE), self-aligned 25 KeV P⁺ implantation at 5×10¹⁵ cm⁻² dose and 900° C. RTA to activate the dopant at source-drain region. The CTF devices were with 10-μm gate length, 100-μm width and isolated by field oxide. The fabricated devices were characterized by cross-sectional transmission electron microscopy (TEM), P/E, cycling and retention measurements to 125° C.

FIGS. 14( a) and (b) show the schematic structure and energy band diagram of the MoN[SiO₂—LaAlO₃]-[Ge—HfON]—[LaAlO₃—SiO₂]—Si CTF device. An ultra-thin ENT of 2.5-nm was obtained from X-TEM. As shown in FIG. 14( a), a flash memory 20 comprises a Si substrate 202, a SiO₂ layer 204 formed on the Si substrate 202, a LaAlO₃ layer 206 formed on the SiO₂ layer 204, a HfON layer 208 formed on the LaAlO₃ layer 206, a Ge/HfON layer 210 (that is formed by reacting Ge with HfON) formed on the HfON layer 208, another LaAlO₃ layer 212 formed on the Ge/HfON layer 210, another SiO₂ layer 214 formed on the another LaAlO₃ layer 212, and a MoN layer 216 formed on the another SiO₂ layer 214. In an embodiment, the flash memory 20 may further comprise another HfON layer formed on the Ge/HfON layer 210, and another Ge/HfON layer formed on the another HfON layer. The energy band at top Ge/HfON may be narrowed due to the smaller bandgap of Ge (0.67 eV) and HfGeON formation. The small bandgap HfGeON lowers the E_(C) that improves the carrier retention. The band offset in LaAlO₃/SiO₂ double tunnel layers reduces the tunneling barrier for faster P/E speeds and better endurance. The high-κ blocking and trapping layers lower the P/E voltage, which in turn improves the erase saturation due to the higher electric field across the thin tunnel SiO₂.

FIGS. 15( a) and (b) show the P/E characteristics of HfON and Ge/HfON CTF, respectively. The V_(th) increases with increasing P/E voltage and time. The larger program V_(th) of Ge/HfON CTF indicates the better trapping capability than that of control HfON device. Small erase saturation was obtained due to the using high-κ dielectrics to give a larger electric field in tunnel SiO₂ for better hole tunneling. Under the ±16 V and 100 μs P/E, the V_(th) difference (ΔV_(th)) between P and E are 2.7 V and 4.4 V for HfON and Ge/HfON CTF devices, respectively. The larger ΔV_(th) memory window of Ge/HfON CTF than control HfON device is due to the Ge reaction with HfON to form the HfGeON that has higher-κ for better erase and higher traps for more efficient charge- trapping.

FIG. 16( a) shows the retention characteristics of HfON and Ge/HfON CTF devices. Under the ±16 V and 100 μs P/E, the 10-year extrapolated retention window at 125° C. for HfON CTF is 1.8 V that largely increases to 3.2 V for Ge/HfON device. Such large 10-year retention window allows multi-level cell (MLC) storage at 125° C. with a record thinnest 2.5-nm ENT. The good retention is due to the deep E_(C) of HfGeON for carrier storage, like the deep-E_(C) of poly-Si FG flash memory. In addition, the larger physical thickness of high-κ-LaAlO₃/SiO₂ double barriers improves the retention. Further, the large energy bandgap and small trap density of SiO₂, formed by CVD TEOS, in double barrier also helps the retention improvement. The fast 100 μs erase speed is due to bandgap-engineered lower hole barrier ΔE_(V) in the LaAlO₃/SiO₂ tunnel oxide. The better trapping capability, faster P/E speed, and lower hole tunnel barrier further lead to excellent endurance as shown in FIG. 16( b), with a larger 10⁶-cycled memory window of 3.6 V than the 2.2 V of control device, at the same ±16 V and 100 μs P/E. Such excellent cycling data are vitally important to allow further endurance improvement in highly scaled CTF device with fewer stored electrons.

The record thinnest 2.5-nm ENT for CTF, large 10-year retention window of 3.2 V at 125° C., and 10⁶ cycled endurance were reached simultaneously under fast 100 μs and low ±16V P/E, which fit the requirements of ITRS shown in Table 2.

TABLE 2 lists a variety of data published by ITRS in 2009. NAND Flash 16 14 13 12 11 9 poly 1/2 Pitch (nm) Cell size 0 4.0/1.0 4.0/1.0 4.0/1.0 4.0/1.0 4.0/1.0 4.0/1.0 area factor a in multiples of F²SLC/MLC Tunnel SiO₂ or SiO₂ or SiO₂ or SiO₂ or SiO₂ or SiO₂ or dielectric ONO ONO ONO ONO ONO ONO material Tunnel 3-4 3-4 3-4 3-4 3-4 3-4 dielectric thickness EOT (nm) Blocking Al₂O₃ Al₂O₃ Al₂O₃ Al₂O₃ Al₂O₃ Al₂O₃ dielectric material Blocking 6 5 5 5 5 5 dielectric thickness EOT (nm) Charge SiN/High-K SiN/High-K SiN/High-K SiN/High-K SiN/High-K SiN/High-K trapping layer material Charge 4-6 4-6 4-6 4-6 3-4 3-4 trapping layer thickness (nm) Gate material Metal Metal Metal Metal Metal Metal Highest W/E 15-17 15-17 15-17 15-17 15-17 15-17 voltage (V) Endurance 1.00E + 04 1.00E + 04 1.00E + 04 1.00E + 04 1.00E + 04 1.00E + 04 (erase/write cycles) Nonvolatile 5-10 5-10 5-10 5-10 5-10 5-10 data retention (years) Maximum 4 4 4 4 4 4 number of bits per cell (MLC)

The foregoing descriptions of the detailed embodiments are only illustrated to disclose the features and functions of the present invention and not restrictive of the scope of the present invention. It should be understood to those in the art that all modifications and variations according to the spirit and principle in the disclosure of the present invention should fall within the scope of the appended claims. 

1-4. (canceled)
 5. A method, comprising: providing a substrate; forming a first SiO₂ layer on the substrate; forming a first high-κ layer on the first SiO₂ layer; forming a metal-implanted oxynitride layer on the first high-κ layer; forming a second high-κ layer on the metal-implanted oxynitride layer; forming a second SiO₂ layer on the second high-κ layer; and forming a gate layer on the second SiO₂ layer.
 6. The method of claim 5, wherein the high-κ layer is Al₂O₃, La₂O₃, HfO₂, ZrO₂, TiO₂, SiN or ternary or quarternary combinations thereof, the oxynitride is AlON, LaON, HfON, ZrON, TiON, SiON or quarternary combinations thereof, and the implanted metal are heavy metals of As, Sb, Ga, or In.
 7. The method of claim 5, wherein the metal-implanted oxynitride is formed by implanting metal at tilted angle and <10 KeV.
 8. The method of claim 5, wherein the gate layer is made of metal or metal-nitride.
 9. The method of claim 8, wherein the metal-nitride is TiN, TaN or MoN. 